The use of pyrolysis tars as a feedstock to make a premium coke suitable for the production of graphite electrodes has long been a goal of the electrode industry. This desire to employ pyrolysis tars, which are the residue by-product of steam-cracking of naphtha or gas oil in the production of ethylene, stems from the fact that pyrolysis tars are in plentiful supply, are low in sulfur content and possess a high coke yield.
In commercial practice, the conversion of hydrocarbon tars to coke is primarily accomplished by the process of delayed coking. The process of delayed coking has long been one of the standard processes for converting low value residual liquid hydrocarbonaceous materials into more desirable products. Essentially this process comprises rapidly heating the feedstock to the desired coking temperature by passing it through a heated conduit before entering it into an insulated, non-heated coke drum where the coking reaction is completed. The coking process is complete when the formation of coke has progressed to the desired extent. Delayed coking is detailed by Charles L. Mantell, Carbon and Graphite Handbook, pp 149-151, Wiley-Interscience (1968) and R. DeBiase, J. D. Elliott and T. E. Hartnett, "Delayed Coking Process Update", Symposium on Petroleum Derived Carbons, Preprints, American Chemical Society, St. Louis Meeting, Apr. 8-13, 1984, pp 412-423.
Unfortunately, however, a major problem associated with the delayed coking of pyrolysis tar is that pyrolysis tars generally contain a large amount of highly reactive molecules. Thus, as has been noted in the prior art (see for example, U.S. Pat. No. 3,817,853), when pyrolysis tars are heated to the necessary temperatures for delayed coking operations coke is prematurely deposited in the heater tubes due to the highly reactive nature of the tar. The extent of this coke formation may be so great that the heating tubes may become plugged in a very short operating time thereby necessitating a shut-down and cleaning of the system.
While U.S. Pat. No. 3,547,804 discloses admixing the pyrolysis tar with a low-boiling distillate to sweep the tar through the heating tubes without premature coke deposition, as noted by U.S. Pat. No. 3,817,853, even when special precautions are taken to avoid premature coke deposition with the highly reactive pyrolysis tars a premium coke (i.e. a coke producing a graphite having a coefficient of thermal expansion, "CTE", of not greater than 0.55.times.10.sup.-6 inch per inch per degree centigrade) may not be produced.
To improve the quality of the coke produced by the delayed coking of pyrolysis tar it has been disclosed in the prior art to hydrogen treat the pyrolysis tar prior to subjecting it to delayed coking. For example, U.S. Pat. No. 3,817,853 discloses pretreating pyrolysis tar by contacting it with hydrogen at a temperature from about 250.degree. to about 800.degree. F. under conditions sufficient to effect a consumption of hydrogen of from 100 to about 2000 standard cubic feet per barrel of pyrolysis tar.
However, while such hydrogen upgrading treatment will produce pyrolysis tars which will yield premium cokes, such treatment is costly, requires a separate hydrogenating unit, and leads to loss in coke yield. Moreover, a potential safety hazard exists in the employment of hydrogen. It would therefore be desirable to have a process for producing premium coke from pyrolysis tar which process does not involve an expensive hydrogenation step with the resultant loss in coke yield.
It is therefore an object of this invention to provide an economical process for the production of premium coke from pyrolysis tar.
It is a further object of this invention to provide a process for the production of premium coke from pyrolysis tar wherein the coke yield of such tar is not adversely affected as in hydrogen pretreatments.
The above and other objects of this invention will become more apparent from the following description and examples.